Variable light-filtering device with a redox compound which functions as its own electrolyte

ABSTRACT

This invention relates to light filters employing redox compounds that are substantially light-transmitting in their oxidized state and capable of forming a stable colored free radical upon the addition of electrons. The colored free radical formed in response to the flow of electric current renders the device light-absorbing. To restore the original lighttransmitting properties, the colored free radical may be readily reoxidized, for example, by reversing the direction of current flow.

OR 3v652t1 9 United 5 Rogers [54] VARIABLE LIGHT-FILTERING DEVICE WITH AREDOX COMPOUND WHICH [451 Mar. 28', 1972 Jonea et al. ..350/l60 Anderaon..350/ 160 Rogera ..350/160 Manor ..350/l60 Klua at al ..3$0/l60 PrimaryExam1nr-Ronald L. Wlbert Arr/nan: Examiner-V. P. McGraw Attorney-Brownand Mikulka Thia invention relate: to

ABSTRACT light flltera employing redox compounda that are aubatantiallyllght-tranatnittlng in their oxidlzed atate and capable cal upon theaddition formed in reaponle to device lightmitting properties,reoxidized, for exa flow.

the

of forming a atable colored lrec radiol electrons. The colored freeradical the flow of electric current rendera the abaorbing.

To mature the original light-tramcolored free radical may be readilymple, by reveraing the direction of current 23 Claim, 11 Drawing FlgureaPATENTEnmza I972 SHEET 1 BF 2 INVENTOR. HOWARD (5. ROGERS fimw n, and mATTORNEYS PATENTEUMAR28 r972 3,652,149

' SHEET 2 OF 2 F l G. 9

40 40 F I G. ll

\ INVENTOR.

HOWARD G ROGERS 6mm amd m ATTORNEYS VARIABLE LIGHT-FILTERING DEVICE WITHA REDOX COMPOUND WHICH FUNCTIONS AS ITS OWN ELECTROLYTE This inventionrelates to light filters and similar devices employing a redox compoundcapable of forming a stable colored free radical upon the addition ofelectrons.

Various systems utilizing an electric current passing through anelectrolytic solution to effect a change in the optical density orspectral absorption characteristics of a substance have heretofore beenknown in the art. One such system involves the alternate plating andunplating of metal at one of the electrodes. Another involves the use ofan electrolytic solution of a reversible color-responsive material whichchanges color or becomes colored upon passing a current through thesolution. Basically, these systems have in common a combination ofelements, viz., a pair of electrodes separated by a suitableelectrolyte, a source of current, and some material or materials whichinitially may be present in the electrolyte and which, upon impressing asuitable current, exhibit a change in optical density or spectralabsorption characteristics, e.g., by plating out or color change fromcolor to colorless or colorless to color.

Such systems generally suffer from at least one of the followingshortcomings: too slow; unstable; non-reversible; not automatically orfully reversible; not uniform or consistent throughout the system,particularly over large areas.

It is therefore the primary object of the present invention to provide anovel system for controlling light transmission.

Another object of the present invention is to provide a variable lightfilter.

Still another object is to provide a variable density window.

Yet another object is to provide non-glare headlamp and windshieldsystems for vehicles.

Another object is to provide variable density light filter lenses toprotect the eyes from brilliant light.

A further object is to provide novel systems for image recordation andimage translation.

A still further object is to accomplish the foregoing objectives byproviding a reversible system utilizing certain redox compounds whosespectral absorption characteristics are altered instantaneously andautomatically upon the addition of electrons in response to the flow ofcurrent from a suitable source and restored upon reoxidation.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the process involving the severalsteps and the relation and order of one or more of such steps withrespect to each of the others and the product possessing the features,properties and the relation of elements which are exemplified in thefollowing detailed disclosure, and the scope of the application of whichwill be indicated in the claims.

It has now been discovered that systems capable of a rapid and uniformchange in spectral absorption characteristics or optical density tocontrol the transmission of light may be obtained by utilizing certainredox compounds, namely, redox compounds that are substantiallylight-transmitting in their oxidized state and capable of forming astable colored free radical upon the addition of electrons whereby theybecome lightabsorbing. By light-transmitting, it is meant that thecompounds are substantially colorless or only faintly colored in theiroxidized state so as to transmit substantially all visible light. Uponthe addition of an electron, they form an intensely colored stable freeradical so as to absorb a substantial portion of light in the visiblerange.

In contrast to prior electrochemical systems, in the present inventionthe aforementioned redox compounds may be used in the absence of anelectrolyte, i.e., they may be used in the absence of an ionizablematerial such as an inorganic salt which, upon impressing an electriccurrent, provides ions which will effect the desired change in spectralabsorption characteristics. Since an electronic rather than an ionicreduction reaction is involved, the conversion to the free radicaloccurs very rapidly in response to the flow of current to provide arapid change in optical density and spectral absorption characteristics.Moreover, the reaction is readily reversible by reoxidizing the freeradical which may be accomplished by simply reversing the direction offlow of the electric current and/or by employing an oxidant which may beoxygen or a suitable oxidizing compound.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a diagrammatic sectional view illustrating one embodiment ofthis invention;

FIG. 2 is a similar view illustrating another embodiment of theinvention;

FIG. 3 is a similar view illustrating yet another embodiment of theinvention;

FIG. 4 is a similar view illustrating still another embodiment of theinvention;

FIG. 5 is a cross sectional view of a lens element prepared inaccordance with the present invention;

FIG. 6 is a plan view illustrating a variable density window prepared inaccordance with the present invention;

FIG. 7 is a parElly schematic vertical sectional view taken i alonglines 7-7 ofFIG. 6;

FIG. 8 is a somewhat schematic sectional view showing the use of thepresent invention in vehicle headlamps;

FIG. 9 is a partly perspective, partly schematic view illustrating theuse of this invention to provide glare-free windshields;

FIG. 10 is spatiall diagrarnrhaticIFI- tiEy sectional viewillustratingthe use of the present invention in elements for preparing visibleimages; and

FIG. 11 is a similar view showing the preparation of a visible imagewith the element of FIG. 10.

In carrying out the present invention, any redox compound may be usedincluding either monomeric or polymeric compounds provided that it issubstantially light-transmitting in its oxidized state and capable offorming a stable, colored free radical upon the addition of electronsand provided that it is capable of being alternately and repeatedlyreduced to the free radical and reoxidized whereby a reversible changein spectral absorption characteristics or optical density is achieved.As discussed above, the redox compound may be substantially colorless inits oxidized state, or it may be faintly colored so as to besubstantially transparent to visible light. Upon the addition ofelectrons, a stable free radical is formed which is deeply colored andlight-absorbing in the visible range.

Examples of redox compounds meeting these criteria are compoundscontaining a strongly delocalized electron, i.e.,

compounds which have an odd number of 1r electrons distributed over aneven number of atoms arranged in a chain. S uch compounds are derivedfrom the general formula X(Y=Y).,X. wherein X is selected from anitrogen. sulfur and oxygen atom; Y is selected from a nitrogen atom andCH: and n is 0 or a positive integer. A discussion of the chemistry ofthese compounds is found in International Union of Pure and AppliedChemistry. Vol. l5 (1967) entitled, International Symposium on FreeRadicals in Solution." pages l09l22.

Redox compounds of this type which have been found especially useful inthe present invention are the so-ealled Weitz radicals as described inthe aforementioned reference where X is nitrogen and Y is CH, such as4,4dipyridylium compounds, diazapyrenium compounds and pyrazidiniumcompounds. Typically, these compounds are substantially colorless toyellow in their oxidized state and upon the addition of an electron forma deeply colored free radical of intense blue to bluish-purple.

Illustrative of suitable 4,4'-dipyridylium compounds are those havingthe formula:

R -i-/ ri-R,

wherein R and R each are selected from hydrogen, alkyl, 5

aralkyl and aryl and A is an anion; and polymers thereof having theformula:

wherein A is defined as above and R is either alkylene having 4 to 8carbon atoms or xylylene and n is at least 2.

Illustrative diazapyrenium compounds are those having the formula:

wherein A, R and R, are defined as above. illustrative pyrazindiniumcompounds are those having the formula:

. wherein A is defined as above.

Typical of the alkyl, aralkyl and aryl substituents are methyl, ethyl,propyl, isopropyl, butyl, hexyl, benzyl, tolyl, pethylphenyl and phenyl.Typical anions include Br, Cl, 50,, C10,, BF, and tosyl.

Specific compounds encompassed by the above formulas are N,N-dimethyl-4,4-dipyridylium dichloride; N,N'-diphenyl-4,4'-dipyridyliumdichloride; N-ethyl-N'-phenyl-4,4- dipyridylium dibromide;N,N'-dibenzyl-4,4-dipyridyliumdiiodide;2,2'-dimethyl-N,N'-diethyl-4,4-dipyridylium dichloride;N,N'-dimethyl-2,7-diazapyrenium difluoroborate;N,N'-diphenyl-2,7-diazapyrenium difluoroborate; N,N'-dibenzyl-2,7-diazapyrenium dibromide; 4,5-dimethyl-N,N'-dimethyl-2,7-diazapyrenium dichloride; dipyrido-[ l ,2-a;2',lc]-pyrazidinium dibromide and 3,3'-dimethyldipyrido-[ 1,2- a;2,l-c]-pyra.zidinium dichloride.

Illustrative of the polymeric 4,4-dipyridylium compounds mentioned aboveare those prepared by reacting 4,4- dipyridyl with an alkylating agent,e.g., 1,8-dibromooctane or 2,2'-dibromo-p-xylene, as described andclaimed in copending application Ser. No. 854,781 of Myron Simon, filedon even date herewith.

Upon the application of the appropriate potential, the aforementionedredox compounds are readily converted to a deeply colored free radical,which is sufirciently stable so that the color once formed will remainover a substantial period of time without further application ofelectric current thereto. In applications where it is desirable toquickly restore the original light-transmitting properties of thecompound, the free radical may be readily reoxidized by reversing thedirection of current flow and/or by using an oxidizing agent. Forexample, an electrically stable organic or inorganic electron donor,such as ceric ammonium nitrate or benzoquinone may be admixed with theredox compound. The oxidizing agent also may be atmospheric oxygen andthe system suitably 75 vented so that oxygen ions from the air canreoxidize the free radical to restore the original spectral absorptioncharacteristics. Where an aqueous solution of redox compound isemployed, the oxygen generated by electrolysis may be used to reoxidizethe colored free radical provided that it does not proceed at a rate tocause excessive bubbling. To control electrolysis, other pathways forthe current may be provided.

In the present invention the aforementioned redox com-\ pounds may beutilized in either a liquid or solid system. They) may be dissolved inaqueous solution or in an electrically stable organic liquid, e.g.,propylene glycol or ethylene glycol monomethyl ether, which additionallymay contain viscosity imparting reagents, such as carboxymethylcellulose. They may be dispersed in a compatible gel, such as gelatin,hydrolyzed polyisopropenyl acetate or hydroxyethyl cellulose, or theymay be dispersed in a compatible polymeric matrix, e.g., polyvinylacetate, polystyrene, cellulose acetate 0 polymethyl methacrylate. Itwill be understood that the liquid or solid used to dissolve or dispersethe compounds will be substantially colorless so as not to interferewith the light transmitting properties of the system. Also, the redoxcom-\ pounds themselves may be used as their own matrix and the solidredox material simply compressed between the electrodes. In systemswhere it is desirable to achieve the greatest possible speed inreversibly altering optical density or spectral absorptioncharacteristics, the redox compound is used in high concentrations andpreferably is used undiluted, i.e., by itself.

In a preferred embodiment, a solid system is employed which comprises afilm of polymeric redox compound made by casting a solution of the redoxpolymer onto a glass plate electrode. If a polarizing material isdesired, the cast film may be stroked in a given direction untiloriented.

As indicated previously, the redox compounds of the present inventionmay be employed without an electrolyte. Sodium chloride, copperperchlorate, acid zinc sulfate, lead nitrate, hydrochloric acid andother ionizable substances commonly used to provide ions to effect colorchange are not essential to the operation of the systems of the presentinvention. The redox compounds will function as their own electrolyteand will exhibit a change in optical density or spectral absorptioncharacteristics in response to an electronic reaction. Ifdesired,however, the redox compounds may be used in an ionic system, Le, aconventional electrolytic system employing the aforementioned ionizablesubstances.

One electrolytic system which has proved especially valuable comprises asaturated solution of a 4,4'-dipyridylium compound in aqueous acidsolution. Using about one-tenth to onethird of the total volume ofhydrochloric acid with N,N'- dimethyl-4,4-dipyridylium dichloride, anoptical density of 3 may be achieved in about microseconds by applying apotential of 500 volts. Besides hydrochloric acid, other organic orinorganic acids may be used as the electrolyte, such as acetic acid,citric acid, sulfuric acid, and nitric acid. Where rapid clearing of thesystem to its light-transmitting characteristics is desired, anoxidizing agent, e.g., ceric ammonium nitrate may be incorporated intothe electrolytic solution. An electrically responsive light-absorbingelement utilizing this and other electrochemical systems is describedand claimed in application Ser. No. 880,796 by Howard G. Rogers and JohnJ. McCann filed Nov. 28, 1969.

It will be appreciated that the aforementioned compounds will exhibitdifierent spectral absorption characteristics at the anode than theywill at the cathode. Oxidation reactions will occur and the pH will beacidic at the anode while reduction reactions will occur and the pH willbe basic at the cathode. Thus, when the compound is positioned adjacentone of the electrodes, the flow of current is controlled so that thiselectrode may be either the anode or the cathode and the spectralabsorption characteristics of the compound will be determined by thedirection of the flow of current.

It will be understood that the above-described redox compounds may beused in conjunction with other oxidationreduction indicators whichrequire the presence of an ionizable material to provide ions foreffecting color change. As is well known in the art, alloxidation-reduction indicators change color at a set electricalpotential. Thus, the DC. potential necessary to obtain the desiredspectral absorption or optical density change will depend upon thepotential required for the particular compound employed and will dependin part on the conductivity of the system and cell geometry. It will beapparent to those skilled in the art that the total voltage required fora given compound and a given arrangement of elements may be ascertainedby reference to the values for electrical potential given in theliterature together with routine experimentation. For greatestefiiciency, the set electrical potential of the above-described redoxcompounds and conventional oxidation-reduction indicators used should befairly similar.

The set electrical potential for various oxidation reduction indicatorsare set forth in the following table wherein the standard value is givenwhen the dye is half oxidized and half reduced.

TABLE P at Color change p Indicator C. Oxidized Reduced Sairauine T 0.24 Blue-violet Colorless. Neutral red... 0. 24 Red-violet Do. Indigomonosulloni 0.26 D0. Phenosafranine O. 28 Do. Indigo tetrasulfonic acid0.36 Do. Nile blue N 0.41 D0. Methylene blue 0.53 D0.l-naphthol-Z-sodium sulfonate- 0. 54 Do.

indophenol. Phenol-indo-2, -dlbromolndo- 0. 67 Red Do.

phenol. Blndschedlers green 0.68 Green. Do. Diphenylamine. 0. 76 VioletD0. Diphenylamine-p-sulfon acid 0. 84 Red-violet Do. Erioglaucine A 1.00Green. Blue-red. Setoglaucine O 1.01 Yellow Yelliwp-Nitrodiphenylamine1.05 Red-violet. Colorless. Diphenylarnine-2,3-dicarboxylic 1. 12 \ioletGreen.

acid.

1. 26 Blue Do.

Diphenylarnine-2,2dicarb0Xylic acid.

For a listing of additional oxidation-reduction indicators and acid-baseindicators which may be used in conjunction with the redox compounds ofthe present invention, reference is made to Chemical Indicators, 0.Tomicek, Butterworths Scientific Publication, 195 l.

The use of the above compounds in the practice of this invention will bemore readily understood by reference to the drawings.

As shown in FIG. 1, the light filter or light transmission controllingmeans designated at 1 comprises a pair of spaced, parallel electrodes 2and 3 which are light-transmitting and a substantiallylight-transmitting redox compound as described above or a mixture ofsuch compounds 4 confined between and in contact with the electrodes. Asdiscussed previously, where it is desired to rapidly clear the system torestore its original light-transmitting properties subsequent to formingthe colored free radical, the direction of current flow may be reversedand/or an oxidizing compound may be used in conjunction with the redoxcompound. Rather than mixing an oxidizing agent with the redox compound,the device may be vented, in any suitable manner to allow oxygen ionsfrom the atmosphere to reoxide the free radical. Where an aqueoussolution of the compound is used, the oxygen resulting from hydrolysismay be employed to clear the system. Depending upon the end use of thefilter and the rate at which it is desired to clear the system, currentreversal and/or one or more oxidizing agents may be used.

The electrodes 2 and 3 are at least translucent and preferably aresubstantially transparent to light. They may be any of thelight-transmitting electrodes heretofore known in the art, e.g.,transparent bases of plastic or glass supporting a thin coating of aconducting metal such as gold, fine mesh screens of a noble metal whichmay be carried on a transparent support and glass coated with atransparent film of an oxide, such as cadmium, indium or tin oxide.Where the electrodes comprise transparent bases having an electricallyconductive surface, they may be used to form opposed end walls of thefilter device, provided they are structurally adequate. Alternatively, aseparate container may be used to enclose the system.

Electrodes 2 and 3 are connected to a suitable source of electriccurrent by leads 5 and 6, respectively. As shown in FIG. 1, the sourceof current may comprise a battery 7. Resistor 8 is preferably but notnecessarily provided in order to regulate the amount of current. Theterminal ends of leads 5 and 6 make contact with double switch 9 tocomplete the circuit.

In operation, when the switch is in the up" position, as shown in FIG.1, the. current is caused to flow from the battery through switch 9a tolead 5 whereby electrode 2 becomes the anode and electrode 3 becomes thecathode. Switch 917 of course makes contact with lead 6 to complete thecircuit. Thus, electrode 2 pemiits oxidation reactions and electrode 3permits reduction reactions. That layer of redox compound adjacent thecathode, i.e., electrode 3 is converted to a colored free radical sothat a colored filter is obtained.

Where the device is to be cleared by reversing the current flow thedirection of current may be controlled manually or it may be controlledautomatically, e.g., by means such as illustrated in the bottom portionof FIG. 1. As is shown, this automatic means may comprise a solenoid 11,photoconductive cell 12, battery or other source of current 13 and aresistor 14 connected in series by means of leads l5, 16, 17 and 18,respectively.

As is well known in the art, cell 12 and resistor 14 cooperate tocontrol the amount of current flowing to solenoid 11. When the currentis of a predetermined amount, e.g. when photoconductive cell 12 recordsa relatively low intensity of light, solenoid 11 causes switch 9 to dropto the down position, thereby reversing the direction of the flow ofcurrent.

By way of illustration, in a system employing conductive glasselectrodes and a dipyridylium compound as the redox material, a changeof color from yellow to blue occurs in the vicinity of electrode 3(cathode) when the switch is in the up" position. Upon reversing thecurrent flow, the original yellowish color is restored to the deeplycolored layer adjacent electrode 3, which upon current reversal becomesthe anode.

In clearing the device by current reversal it has been found that thebest results are achieved by using a slightly lower potential forreoxidizing the colored free radical than the potential needed for thereduction reaction. To reduce a given redox compound to its colored freeradical, a certain minimum voltage is required depending upon the setelectrical potential of the compound and upon the cell used, i.e., thegeometry, the particular electrodes employed, etc. While the coloredfree radical formed may be reoxidized simply by reversing the directionof current flow and applying the same potential, a colored layer ofredox compound tends to form adjacent the opposite electrode whichbecomes the cathode upon switching the polarity of the system. Theformation of color at the opposite electrode may be prevented and thecell visibly cleared by applying a voltage slightly below, e.g., about0.1 volt below the minimum needed for the reduction reaction. It will beappreciated that the speed with which the compound is reduced to itscolored form is in large measure dependent upon the potential applied.Thus, for achieving very rapid color formation, the voltage applied isconsiderably above the minimum.

In the filtering device described above, only the aforementioned redoxcompounds are employed. It is also within the scope of the invention touse the aforementioned redox compounds adjacent one of the electrodesand another compound such as a pH or acid-base indicator adjacent theother electrode.

Such a filtering device is illustrated in FIG. 2 wherein a redoxcompound 4, such as a 4,4'-dipyridylium compound, is

adjacent electrode 3 and another material having reversibly alterablespectral absorption characteristics 40 is adjacent electrode 2.Materials 4 and 40 are separated by a light-transmitting selectivelypermeable membrane 19, such as cellophane, which prevents migration andadmixing of the materials 4 and 40 while permitting the flow of current.The membrane may be eliminated in solid systems where materials 4 and 40are in a gel or other non-liquid form.

Material 40 may comprise a compound whose spectral absorptioncharacteristics are determined by the pH of the environment, e.g.,so-called pI-I or acid-base indicators. Illustrative of such compoundsare phenolphthalein which changes from colorless in an acid medium topink in a basic medium; Malachite Green which changes from green tocolorless; Phenacetolin which changes from red to colorless; bromocresolgreen which changes from yellow to blue; etc.

Another class of compounds useful as material 40 are those which arecapable of being alternatively or reversibly reduced and re-oxidized andwhich exhibit spectral absorption characteristics in the reduced statedifferent from those exhibited in the oxidized state. Examples of suchcompounds are leuco dyes which typically are colorless in the reducedstate and colored in the oxidized state and oxidation-reductionindicators such as Phenosafranine which changes from blue in anoxidizing environment to colorless in a reducing environment; Indigotetrasulfonate which changes from blue to colorless; diphenylamine whichchanges from violet to colorless; Erioglaucin A which changes from greento red, etc.

Other compounds useful as material 40 are polymers which exhibitdifi'erent spectral absorption characteristics in an acidic versus analkaline environment Examples of such polymers are S-nitrosalicyaldehydepartial acetal of polyvinyl alcohol which changes from colorless toyellow in basic medium; 3'-formyl-phenolphthalein partial acetal ofpolyvinyl alcohol which changes from colorless to red in basic medium;etc.

It will be appreciated that material 40 may be dissolved in a solutionor dispersed in a gel containing any of the common electrolytes. Also,material 40 may possess or be capable of possessing the same ordifferent spectral absorption characteristics, e.g., the same color or acolor different from material Using two separate materials in thismanner allows more efficient utilization of a given voltage than ispossible with a filtering device as illustrated in FIG. 1. For example,a blue filter, i.e., one which is either blue or substantiallytransparent, depending on the flow of current may be obtained using a4,4'-dipyridylium compound, such as N,N-dimethyl-4,4'- dipyridylium(methyl viologen) as material 4 and the oxidation-reduction indicator2,6-dibromophenyl indophenol as material 40.

Methyl viologen is substantially colorless to faintly yellow in itsoxidized state and an intense bluish color when reduced. Conversely, theindophenol is blue in its oxidized state and substantially colorlesswhen reduced. Thus, when current is impressed so that electrode 2 is theanode and electrode 3 is the cathode, both materials 4 and 40 becomeblue to provide a blue filter, the light transmission characteristics ofwhich are a function of both materials. When the current flow isreversed so that electrode 2 is the cathode and electrode 3 is theanode, the device 1 becomes substantially light-transmitting.

Rather than using the indophenol oxidation-reduction indicator, one mayuse a pH indicator such as the disodium salt of 5,5'-indigodisulfonicacid, which will be blue in an acidic environment as when electrode 2 isthe anode and substantially colorless in a basic environment as whenelectrode 2 becomes the cathode upon reversal of the current flow. Also,it will be appreciated that where materials 4 and 40 are in a solid formthat the membrane 19 is not essential. If desired, material 4 maycontain an oxidizing agent.

In the filtering devices previously described, the electrodes are shownto be in parallel relationship with one another. In lieu of thisarrangement, the electrodes may be positioned in different relationshipwith one another and/or may have differ ent shapes. For example, theelectrodes may be perpendicular to one another; or one electrode may becoiled around the other electrode. A gross electrode comprising a flat,rectangular light-transmitting electrically conducting plate may be usedwith a minor electrode comprising a circular or U-shaped rod or wirepositioned outside of the useful optical path of the device. In suchvariations, suitable transparent means may be employed to confine thelight-filtering compound.

In the embodiments heretofore described, a pair of electrodes have beenemployed. However, it is within the scope of this invention to providevariable filters employing more than two electrodes.

In FIG. 3, there is shown a variable flter having three electrodes, twoof the electrodes being in parallel relationship with one another, thethird being substantially perpendicular to the first two electrodes andoutside the field of light-transmittancy of the filter but in contactwith the light filtering materials 4 and 40.

As in the embodiment illustrated in FIG. 1, leads 5 and 6 are connectedto a suitable source of current. Lead 6a connects electrode 30 to lead6, so that electrodes 3 and 3a will be of the same potential whencurrent is impressed. Electrode 2, which need not be transparent as itlies beyond the field of light-transmittancy of the filter, is connectedby means of lead 5 to the current source.

In one embodiment, materials 4 and 40 are identical in their spectralabsorption characteristics so as to provide a variable filter similar insome respects to the filter of FIG. 2. For example, bromothymol bluewhich is blue in a basic environment and yellowish in an acidenvironment may be employed as material 40 with methyl viologen asmaterial 4 which is blue in its reduced fon'n and colorless to faintlyyellow in its oxidized form. When current is impressed in one direction,both electrodes 3 and 3a are cathodes so that materials 4 and 40 areblue. When the direction of current flow is reversed both electrodes 3and 3a become anodes, so that both materials 4 and 40 becomesubstantially transparent.

By means of suitable wiring, switches and the like (not shown), eitherof electrodes 3 and 3a may be disconnected, so that current may becaused to flow to only one of the electrodes 3 and 3a. Thus, it ispossible to provide a filter of varying density merely by connecting ordisconnecting one of the electrodes. For example, if lead 6a isdisconnected, the filtering ability of the system will be a functiononly of the density of the filter produced at electrode 3. Byreconnecting lead 6a, the density is increased due to the lightabsorption now provided at electrode 3a. Obviously, the connecting anddisconnecting may be controlled either manually or automatically.

Where materials 4 and 40 are different, e.g., when they aresubstantially colorless in one environment and exhibit differentspectral absorption characteristics in another environment, threedifferent colored filters may be obtained. Where both electrodes 3 and3a are connected so that materials 4 and 40 exhibit a color, theresulting filter will be a product of the absorption characteristics ofboth materials. Alternatively, either electrode may be disconnected toprovide a filter of the absorptive characteristics of thecolor-providing material associated with the respective electrodes 3 and3a.

It is also contemplated that both the materials 4 and 40 may be the sameor different redox compounds of the present invention, e.g., a4,4-dipyridylium and/or diazapyrenium compound described above.Alternatively, a redox compound, e.g., a 4,4'-dipyridylium compound, maybe used as material 4 in this arrangement without another colorchangingmaterial 40. When both materials 4 and 40 are used and are in a gel orother solid form, membrane 19 may be eliminated.

As mentioned previously, in a preferred embodiment of the presentinvention, the light-filtering redox compound is preferably a polymericcompound, such as, the 4,4'-dipyridylium polymers described above.Polymer films of this type, including molecularly oriented polymer filmsmay be used as materials 4 and 40 in the filtering device of FIG. 3.When oriented films are employed, they may be positioned with their axesof polarization at a predetermined desired angle, for example, at rightangles for potential total extinction of light. When current isimpressed so that electrodes 3 and 3a are cathodes, a pair of deeplycolored blue light polarizers are formed having their axes, for example,at right angles thereby permitting a minimal amount of light to betransmitted through the filtering device. When the current fiows in theopposite direction, the oriented films lose their polarizing propertieswith respect to visible light, and the films change from blue to theiroriginal yellowish color. By employing suitable wiring the variablefilter of FIG. 3 is capable of providing a pair of polarizers or asingle polarizer at either of the electrodes 3 or 3a.

Another embodiment comprising a polarizing system is illustrated in FIG.4 wherein a permanent polarizer 40 is provided on the side of electrode2 opposite an oriented dipyridylium polymer film 4. Polarizer 40 alsomay be placed on the other side of the electrode 2 provided that it willallow the passage of current and is electrically stable. The permanentlight-polarizing sheet 40 may also be positioned on the outer surface ofelectrode 3. As in the above embodiment, the layer of polymeric material4 adjacent electrode 3 forms a blue light polarizing film upon theaddition of electrons and upon reoxidation reverts to its originalyellowish color. Reoxidation may be achieved by using an oxidizing agentand/or by reversing the direction of current flow.

In view of the foregoing illustrative embodiments, other variations inthe structure and/or the arrangement of elements of the novel filteringdevice of this invention will be readily suggested to those skilled inthe art.

The light-filtering devices of this invention are capable of a varietyof different uses in which a stable, unifonn instantaneously variablelight filter is desirable or necessary. They may be employed, forexample, in lens elements for goggles or shields for protecting the eyesfrom bright light. A typical lens element is shown in FIG. 5.

As shown therein, the element comprises a transparent lens havingopposed faces and the novel filtering device 1 as shown, for example, inFIG. 1 in juxtaposition with and coextensive with one faceof the lens.The filter l is connected by leads (not shown) to a suitable source ofcurrent. The filter 1 may be bonded to the lens 10 using any suitablelighttransmitting adhesive, e.g., an epoxy resin.

It will be appreciated that a single such lens element may be used as anoverall shield or two such elements may be used to protect the eyes andthe element(s) mounted and cemented in a suitable frame provided withopenings to accommodate the leads for connecting the filter element to asource of current.

The novel filters of the present invention also find utility in variabledensity windows for controlling the amount of light entering a room orother enclosure. A typical variable density window is illustrated inFIGS. 6 and 7.

As shown therein, the variable density window comprises a recessed orgrooved frame or mount of a suitable non-conductive material such aswood confining the novel filtering device 1 as shown, for example, inFIG. 2 between a pair of transparent non-conducting plates 21. Thefilter l is connected to a suitable source of current by leads (notshown) passing through frame 20.

Where found expedient or desirable to do so, filter l and plates 21 maybe laminated together at the top and bottom by means of a suitablebonding material to provide a unitary structure. Suitable bondingmaterials such as epoxy resins, vinyl acetate resins, etc. will bereadily suggested to those skilled in the art. It is also contemplatedthat a polarizing filtering device may be used, such as shown in FIG. 3,wherein materials 4 and 40 comprise a molecularly oriented 4,4-dipyridylium polymer film. Also, a pair of polarizing or nonpolarizingfiltering devices, each with its own protective plate or plates, may beprovided on either side of frame 20 in which event the two devices mayhave an air space therebetween.

In controlling the amount of light entering a room or other enclosure, amaximum amount of current may be generated to provide a variable filterwith maximum light absorption characteristics in response to maximumbrightness. Conversely, in minimal brightness the flow of current may beautomatically reversed to render the filtering device instantaneouslylight transmittant. Rather than reversing the current, reoxidizing toclear the system may be accomplished by chemical means using anoxidizing agent. For example, the system may be suitably ventilated toallow the redox material to be reoxidized in air.

The present invention is also useful in systems for providing non-glareand fog-penetrating headlamps for vehicles. It is contemplated that thepresent invention may be employed in automobiles and the like where theheadlamps may be instantaneously rendered non-dazzling and/0rfog-penetrating by applying an electric current in a given direction.When the direction of flow of current is reversed, the headlamp permitslight to pass in muchthe usual way. The vehicle battery may be employedas the source of current and the direction of current flow may becontrolled either manually by the occupant or automatically by aphotoelectric cell or the like adapted in the manner illustrated in FIG.1, to reverse the flow of current when motivated by the light from anoncoming vehicle.

FIG. 8 illustrates somewhat schematically a headlamp of this invention.As shown therein, a typical headlamp 22 has a light bulb 23, a reflector24 and a transparent protective cover 25 of glass or the like. Thevariable light filter 1, such as shown 7 in FIG. 1, is positioned atsome point between bulb 23 and cover 25 and mounted in a frame 26 or thelike. Frame 26 is provided with a suitable opening through which leads 5and 6 are connected to a suitable external source of current, e. g., the

vehicle battery, although the headlamp assembly may be provided with itsown source of current if desired. Also, it will be appreciated that thedevice 1 need not be behind the cover 25 nor that cover 25 is essentialbut that the arrangement shown affords protection for the assembly.

In use, the headlamp is normally substantially similar to conventionalheadlamps in terms of the transmittance of light emitted from bulb 23.In other words, with a variable filter such as illustrated in FIG. 1,the redox material 4 is substantially transparent until the current iscaused to flow through lead 5 to electrode 2 so that electrode 3 becomesthe cathode and material 4 becomes colored to render the lighttransmitted non-dazzling. When oriented polymer films are employed, thematerial 4 is light-polarizing upon changing color. Upon reversal of theflow of current, material 4 once more becomes substantiallylight-transmitting and non-polarizing to visible light so that lightemitted from bulb 23 is again transmitted in much the conventionalmanner.

The present invention is also particularly useful in polarizing systemsfor providing glare-free windshields. Systems employing polarizingWindshields to protect the occupants of a vehicle from the glare ofoncoming headlights are well known in the art and, are disclosed, forexample, in U. S. Pat. Nos. 2,031,045; 2,087,795; and 2,440,133.

Essentially such systems employ a polarizer in the windshield and apolarizer in the headlamps, each having the same axis of polarizationforming approximately a 45 angle with the vertical.

With two vehicles equipped in this manner approaching from oppositedirections, the headlamp polarizer of one vehicle would have its axis ofpolarization perpendicular to that of the windshield polarizer of thesecond vehicle.

In this case, the light from ones own headlamps would be visible throughthe windshield, while the glare from oncoming headlights would beeliminated. However, a windshield provided with a polarizer in themanner heretofore known in the art has the inherent disadvantage oftending to obscure vision in certain instances by decreasing theintensity of transmitted radiation at times when the maximum intensityof transmitted radiation is desirable.

The present invention obviates this inherent disadvantage by providing asystem whereby a normally transparent windshield may be renderedlight-polarizing only at desired intervals, e.g., when it is desirableto prevent glare from oncoming headlights.

FIG. 9 illustrates the use of this invention in anti-glare systems forvehicles.

A vehicle 30 has its windshield 31 provided with a filter device such asillustrated in FIG. 1 having a molecularly oriented film of a redoxcompound, e.g., a 4,4'-dipyridylium polymer, capable of exhibiting lightpolarizing properties when current is impressed in a given directionactuated automatically by the intensity of light emanating from theheadlights of an oncoming vehicle. The headlamps 22 also are providedwith polarizing units which may be the conventional polarizersheretofore suggested for such usage. The polarizing units also maycomprise the reversible polarizing systems of the present invention. Theaxes of polarization of the respective polarizers are the same and areapproximately at a 45 angle to the vertical as shown by the diagonallines.

The windshield 31a of a second vehicle equipped in similar manner wouldhave a variable filter having an axis of polarization (viewed fromwithin) as shown by the diagonal lines. In other words, the axis ofpolarization of the windshield of one vehicle will have an axis ofpolarization perpendicular to that of the headlamps (and windshield) ofa second vehicle ap-' proaching from the opposite direction.

In the manner heretofore described, the light emanating from theapproaching headlamps and striking the photoconductive cell of thefiltering device would automatically and instantaneously cause the redoxmaterial in the light filter in the windshield element to become deeplycolored and light polarizing and thus, eliminate glare from the oncomingvehicle. When the vehicle has passed, the process is automatically andinstantaneously reversed whereby the redox material returns to itsoriginal light-transmitting condition.

It is also contemplated that the variable filter device associated withthe windshield may also contain at least a second layer of redoxmaterial capable of providing a polarizer with its axis of polarizationvertical, so as also to reduce glare from sunlight reflected off theroad surface. In such a system, the variable filters of FIGS. 2-4 areparticularly useful. It will be understood that the non-polarizingsystems of the present invention also may be used to provide non-glareauto vehicle systems.

The present invention is also useful in photographic processes for imageformation or translation, particularly in the field of documentduplication.

FIGS. and I1 illustrate this aspect of the present invention. As shownin FIG. 10, a photographic unit of this invention may comprise, inorder, a first transparent electrode 2, a photoconductive layer 27, aredox material 4, such as the 4,4- dipyridylium compounds describedabove, and a second electrode 3.

Photoconductive layer 27 comprises a layer of a photoconductivematerial, e.g., a material which is rendered electrically conductive inits transverse direction only upon exposure to light. Materials havingthis characteristic and their preparation are well known in the art andper se comprise no part of the present invention. Examples of suchmaterials include cadmium sulfide, selenium, etc.

In FIG. 11 which illustrates the use of the photographic unit of FIG.10, a document or other object 29 to be reproduced having transparentareas 29a is placed between a suitable source of light 28 and thephotographic unit. Light passing through areas 29a render correspondingareas 270 of photoconductive layer 27 electrically conductive. Thisimagewise conductivity is employed to provide a visible image in thefollowing manner.

When electric current is impressed, an imagewise change in the spectralabsorption characteristics occurs in areas 4a of redox material 4.Material 4 is initially light-transmitting so that areas 4a becomecolored upon impressing the current to provide negative image.Subsequent to image formation, the current either may be left on orswitched ofi" and the resulting image viewed by reflection throughtransparent electrode 3.

Where material 4 is a molecularly oriented redox polymer as describedabove, a polarizing image is obtained. When the polarizing imageprepared in this manner is of such low con- 5 trast as to be faint, itmay be viewed through a polarizer or analyzer, the polarizing axis ofwhich is crossed with that of the polarizing surface on which the imageis formed. In a known manner stereoscopic images also may be formed bysuperimposing a pair of polarizing images with the polarizing axis ofone at right angles with the polarizing axis of the other. Where thepolarizing images are of high contrast, they are particularly useful inadvertising displays and the like which rely for appeal on unusualoptical effects.

Various changes may be made in the structure of the photographic unit ofFIG. 10 without departing from the scope of the invention. For example,it is contemplated that a metal layer could be placed on the back of thephotoconductive layer to facilitate viewing of the image by reflection.Also, a light-absorbing element could be placed between the layer ofredox material 4, and the photoconductivelayer 27 to prevent unwantedchanges in conductivity of layer 27 resulting from viewing lightentering the back of the photographic unit through electrode 3.

The images prepared in the foregoing manner may be erased by subjectingphotoconductive layer 27 to an overall exposure and continuing the fiowof current in the same direction. The overall exposure will renderphotoconductive layer 27 uniformly conductive which in turn causes auniform change in spectral absorption characteristics of redox material4. The colored material may be reoxidized to its colorless form byexposure to atmospheric oxygen or by using an oxidizing compound inadmixture with the redox material. Alternatively, the originallight-transmitting properties of the redox compound may be restored bysimply reversing the direction of current flow.

Once the image has been erased," a new image may be fonned and the unitused repeatedly. Also, the images produced may be permanently recordedusing conventional or difi'usion transfer photographic techniques beforethey are erased.

In the foregoing description of the many uses of the novel filters ofthis invention, a single such filter has been employed.

It is contemplated that a plurality of such filters may be employed, ifdesired. For example, two or more filters may be connected in series toprovide increased filtering capabilities.

The following examples are given to further illustrate the presentinvention and are not intended to limit the scope thereof.

EXAMPLE 1 A light-filtering cell was constructed by cementing alongthree edges a pair of spaced, parallel rectangular conductive glasselectrodes. The light-transmitting conducting tin oxide coatings of theelectrodes were positioned so as to face each other, and the electrodeswere connected to a source of DC current. The space between theelectrodes was filled with a saturated solution ofN,N'-dimethyl-2,7-diazapyrenium difiuoroborate in water. The solutionwas light orange in color and after applying a potential of 3 volts DCacross the cell, the layer of solution in the vicinity of the cathodeturned green. The application of current was discontinued and thesolution allowed to clear to its original orange color upon exposure toatmospheric oxygen.

EXAMPLE 2 Using the apparatus of Example 1, the cell was filled with asaturated solution of dipyrido[ l,2-a;2',l',-c]-pyrazidinium dibromidein water. The solution was pale in color and upon applying a potentialof 3 volts DC across the cell, the solution in the vicinity of thecathode turned yellow-brown. After discontinuing current flow, thesolution cleared to its original pale color upon exposure to atmosphericoxygen.

EXAMPLE 3 Example 1 was repeated using a saturated solution of N,N-dimethyl-4,4-dipyridylium dichloride which was faintly yel- EXAMPLE 4Example 3 was repeated using N,N'-dimethyl-4,4'- dipyridylium dichlorideexcept that a potential in excess of 2.3 volts DC was applied in onedirection across the cell whereupon the layer of solution adjacent thecathode turned from yellow to a deep blue-purple, and then the directionof current flow was reversed and a lower potential of 2.1 volts appliedwhereupon the solution cleared to its original yellow color.

EXAMPLE 5 Example 1 was repeated except that a saturated solution ofN,N'-dibenzyl-4,4'-dipyridylium dichloride in water was employed. Thesolution turned a deep blue in the vicinity of the cathode when apotential of 3 volts DC was applied across the cell. The flow of currentwas discontinued and the solution allowed to stand in air whereupon itcleared to its original yellow color.

EXAMPLE 6 Example 1 was repeated except that a saturated solution ofN,N'-dimethyl-4,4'-dipyridylium dichloride in ethylene glycol monomethylether was employed in the cell. The solution changed from yellow to deepbluish-purple adjacent the cathode when a potential of 2.5 volts DC wasapplied. The solution cleared to its original yellow color upon standingin air after current flow was discontinued.

EXAMPLE 7 Example 1 was repeated except that the cell was filled with asaturated solution of N,N-dimethyl-4,4'-dipyridylium dichloride inaqueous hydrochloric acid electrolyte which contained 0.2 cc. of asaturated aqueous solution of ceric ammonium nitrate per 16 cc. ofelectrolyte. The electrolyte contained 50 percent by volume acid.

Upon applying a potential of 2.3 volts DC across the cell, the solutionadjacent the cathode turned an intense blue-purple. Substantiallysimultaneously with discontinuing current flow, the solution cleared toits original yellow color.

EXAMPLE 8 A layer of dry poly-N,n-butylene-4,4"dipyridylium dibromidewas compressed between a pair of transparent conductive glass electrodeshaving their conductive tin oxide coatings facing inwardly andcontiguous with the opposed surfaces of the polymer layer. Theelectrodes were connected to a source of DC current and a potential inexcess of 1.5 volts was applied across the cell. Upon applying thecurrent, the layer of polymer adjacent the cathode changed from paleyellow to an intense bluish-purple. After the flow of current wasdiscontinued, the polymeric layer cleared to its original yellow colorupon exposure to atmospheric oxygen.

The poly-N,n-butylene-4,4-dipyridylium dibromide,

used in the above assembly was prepared as follows. A solution of 3.9gm. 4,4dipyridyl and 5.4 gm. of 1,4-dibromobutane in 175 of dryZ-methoxy ethanol was refluxed and stirred under nitrogen for 5 hours.The precipitated product was filtered off while hot, washed withZ-methoxy ethanol and then acetone. The product was dried in vacuo.Product yield was 2.5 gm.

EXAMPLE 9 The polymer prepared in Example 8 was spread on the conductivetin oxide surface of a conductive glass plate electrode and stroked inone direction with polished wood until it was clear. A secondtransparent conductive glass electrode was positioned on the film withits conductive surface contiguous EXAMPLE 10 The polymer prepared inExample 8 was admixed with gelatin to give a blend containing aboutpercent by weight polymer and 20 percent by weight gelatin. Theresulting mixture was spread in a thin film on the conductive tin oxidesurface of a conductive glass plate electrode, and as the secondelectrode, a light-transmitting layer of gold was evaporated on the freesurface of the polymer blend. The electrodes were connected to a sourceof D. C. current, and a potential in excess of 1.5 volts was appliedacross the cell whereupon the film changed color from yellow to blue inthe vicinity of the cathode. After current flow was discontinued, thefilm cleared upon standing in air to its original color.

Since certain changes may be made in the above apparatus, product andprocess without departing from the scope of the invention hereininvolved, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

What is claimed is:

1. A light-filtering device comprising a layer of a light-transmittingredox compound (a) capable of functioning as its own electrolyte and (b)capable of forming a stable colored free radical upon the addition ofelectrons and means for flowing an electric current in a predetermineddirection through said layer, said redox compound being the soleelectrolyte.

2. A variable light-filtering device comprising a pair of spaced,parallel, coextensive light-transmitting electrodes, a layer of alight-transmitting redox compound (a) capable of functioning as its ownelectrolyte and (b) capable of forming a stable colored free radicalupon the addition of electrons confined between said electrodes andmeans for flowing an electric current in a predetermined directionthrough said device, said redox compound being the sole electrolyte.

3. A device according to claim 2 wherein said layer contains anoxidizing agent.

4. A device according to claim 2 including means for reversing thedirection of current flow.

5. A device according to claim 4 including means for automaticallypredetermining said direction of current flow.

6. A device according to claim 2 wherein said layer comprises a solutionof said redox compound in a light-transmitting solvent.

7. A device according to claim 6 wherein said solvent is water.

8. A device according to claim 6 wherein said solvent is an electricallystable organic liquid.

9. A device according to claim 2 wherein said layer comprises said redoxcompound in a light-transmitting solid matrix.

10. A device according to claim 9 wherein said matrix is a gel.

11. A device according to claim 9 wherein said matrix is a polymer.

12. A device according to claim 2 wherein said redox compound is apolymer.

13. A device according to claim 2 including a third electrode in contactwith said layer of compound and positioned outside the field oflight-transmittancy of said pair of lighttransmitting electrodes, andmeans for maintaining said pair of light-transmitting electrodes at thesame potential and said third electrode at a different potential.

14. A device according to claim 2 wherein said compound is adiazapyrenium compound.

15. A device according to claim 14 wherein said compound isN,N'-dimethyl-2,7-diazapyrenium difluoroborate.

16. A device according to claim 2 wherein said compound is apyrazidinium compound.

17. A device according to claim 16 wherein said compound is dipyrido[l,2-a;2,1'-c]-pyrazidinium dibromide.

18. A device according to claim 2 wherein said compound is a4,4'-dipyridylium compound.

19. A device according to claim 18 wherein said compound isN,N-dimethyl-4,4-dipyridylium dichloride.

20. A device according to claim 18 wherein said compound isN,N'-dibenzyi-4,4-dipyridylium dichloride.

21. A device according to claim 18 wherein said compound ispoly-N,n-octylene-4,4'-dipyridylium dibromide.

22. A device according to claim 18 wherein said compound ispoly-N,n-butylene-4,4-dipyridylium dibromide.

23. In an enclosure provided with transparent means for transmittinglight from an external source into said enclosure, the improvement whichcomprises positioning in coextensive relationship with said transparentmeans, a light-filtering device as defined in claim 2.

l i t k

2. A variable light-filtering device comprising a pair of spaced, parallel, coextensive light-transmitting electrodes, a layer of a light-transmitting redox compound (a) capable of functioning as its own electrolyte and (b) capable of forming a stable colored free radical upon the addition of electrons confined between said electrodes and means for flowing an electric current in a predetermined direction through said device, said redox compound being the sole electrolyte.
 3. A device according to claim 2 wherein said layer contains an oxidizing agent.
 4. A device according to claim 2 including means for reversing the direction of current flow.
 5. A device according to claim 4 including means for automatically predetermining said direction of current flow.
 6. A device according to claim 2 wherein said layer comprises a solution of said redox compound in a light-transmitting solvent.
 7. A device accordiNg to claim 6 wherein said solvent is water.
 8. A device according to claim 6 wherein said solvent is an electrically stable organic liquid.
 9. A device according to claim 2 wherein said layer comprises said redox compound in a light-transmitting solid matrix.
 10. A device according to claim 9 wherein said matrix is a gel.
 11. A device according to claim 9 wherein said matrix is a polymer.
 12. A device according to claim 2 wherein said redox compound is a polymer.
 13. A device according to claim 2 including a third electrode in contact with said layer of compound and positioned outside the field of light-transmittancy of said pair of light-transmitting electrodes, and means for maintaining said pair of light-transmitting electrodes at the same potential and said third electrode at a different potential.
 14. A device according to claim 2 wherein said compound is a diazapyrenium compound.
 15. A device according to claim 14 wherein said compound is N, N''-dimethyl-2,7-diazapyrenium difluoroborate.
 16. A device according to claim 2 wherein said compound is a pyrazidinium compound.
 17. A device according to claim 16 wherein said compound is dipyrido(1,2- Alpha ;2'',1''-c)-pyrazidinium dibromide.
 18. A device according to claim 2 wherein said compound is a 4, 4''-dipyridylium compound.
 19. A device according to claim 18 wherein said compound is N, N''-dimethyl-4,4''-dipyridylium dichloride.
 20. A device according to claim 18 wherein said compound is N, N''-dibenzyl-4,4''-dipyridylium dichloride.
 21. A device according to claim 18 wherein said compound is poly-N,n-octylene-4,4''-dipyridylium dibromide.
 22. A device according to claim 18 wherein said compound is poly-N,n-butylene-4,4''-dipyridylium dibromide.
 23. In an enclosure provided with transparent means for transmitting light from an external source into said enclosure, the improvement which comprises positioning in coextensive relationship with said transparent means, a light-filtering device as defined in claim
 2. 